The exhaust gases from a vehicle comprise certain contaminants which it is necessary to reduce, indeed even eliminate, before discharging the gases to the atmosphere. The standards are increasingly strict and the tolerated content of contaminants in exhaust gases emitted by vehicles is increasingly low. They are in particular nitrogen oxides, generally known under the name NOx, comprising in particular nitrogen oxide NO and nitrogen dioxide NO2. There currently exists, on the exhaust line of some vehicles, a unit known as catalytic converter, the role of which is to reduce the content of NOx compounds in exhaust gases, indeed even to eliminate all or a portion of these compounds, before the ejection of the exhaust gases into the atmosphere. For this, a catalytic converter used is known under the name of selective catalytic reduction (SCR) device. A reducing agent is generally injected into the exhaust line of the vehicles, upstream of the selective catalytic reduction device, which reducing agent is an active product in the form of gaseous ammonia or a product which is a source of the active product, for example water and urea, for example sold under the name Adblue®, DEF or others, the reaction of which with the exhaust gases triggers, with the evaporation of the water, a thermolysis and a hydrolysis of the urea under certain thermal conditions to form an active product composed of ammonia and isocyanic acid, the latter being obtained if the conversion into ammonia is incomplete. The ammonia and the isocyanic acid, if appropriate, by reaction in the catalytic converter with the exhaust gases and very particularly with the nitrogen oxides which they comprise, form water vapor and molecular nitrogen at the outlet, the isocyanic acid optionally existing at the inlet of the catalytic converter being converted into ammonia and carbon dioxide gas in the catalytic converter by reaction with the water vapor.
The injection of a product which is a source of ammonia, in the example the mixture of water and urea, is carried out by means of a specific injector positioned in the exhaust line sufficiently upstream of the catalytic converter for the ammonia to have the time to be formed on contact with the exhaust gases and to be thoroughly mixed before the source product enters the catalytic converter. A probe at the outlet of the catalytic converter, sensitive to nitrogen oxides, makes it possible to evaluate the effectiveness of the catalytic converter and to adjust the amount of source product injected, via a loop control managed by means of the electronic unit for controlling the catalytic device.
This probe at the outlet of the catalytic converter is also sensitive to the active product and detects the ammonia which might escape from the catalytic converter, an escape which there are generally good reasons to avoid in normal use of the vehicle, and the control unit of the catalytic device normally makes sure that the ammonia content at the outlet of the catalytic converter is reduced to zero. To this end, the selective catalytic reduction device generally has the ability to store a certain amount of buffer ammonia in order to improve its effectiveness at low temperatures, for example less than 200° C. This amount of buffer ammonia is stored by adsorption in the catalytic converter, with a maximum level of storage which falls when the temperature increases and which depends in addition on other conditions, such as the flow rate by volume, the NH3 content in the exhaust gases, and the like. If these conditions change, NH3 can be desorbed and appear at the outlet of the catalyst. The control unit of the selective catalytic reduction device continually manages an appropriate ammonia storage content in the selective catalytic reduction device, determined by a model, and/or an NOx content at the outlet of the catalytic device which is as low as possible without the presence of ammonia in the exhaust gases. This management is carried out in a loop by means of the data transmitted by the probe at the outlet of the catalytic converter.
It may happen that the injector of the active product or of the product which is a source of the active product drifts in the course of use, leading to the opening time of the injector being increased in order to obtain the same effectiveness in terms of reduction of the NOx compounds at the outlet of the catalytic device, this with a limit given by a drift which is such that the correction no longer makes it possible to sufficiently reduce the content of NOx compounds.
The source product is generally composed of 32.5% urea and 67.5% water, which represents the optimum amount of urea for this source product to remain liquid at the lowest temperatures possible. The control unit of the catalytic device is prepared on the basis of a given source product. It may also happen that the percentage of urea present in the source product varies, resulting in additional difficulties in controlling and obtaining a sufficient reduction in the NOx compounds at the outlet of the catalytic device.
It may also happen that the catalytic converter is of poor quality and/or experiences premature ageing.
The control loop is thus subject to all the sources of drifts of the system (drift of the injector, poor quality of the active product, catalyst and/or sensor failing, and the like) which may impact the conversion of the NOx compounds. The control loop will, to a certain extent, be able to compensate for these sources of drifts.
OBD (On-Board Diagnostic) legislation requires that this conversion of NOx compounds be monitored and that a warning be given in the event of dysfunctioning. In the event of dysfunctioning, it is thus currently impossible to determine from where the problem more exactly arises.